This is not a great start to my first orienteering session. My phone has completely lost any signal, the carefully plotted co-ordinates of the meeting point have vanished from Google Maps, and I can't even find the car park. Fortunately, Southdowns Orienteers are far more organised – I spot a sign stuck to a tree. I think we are in business.

Despite my appetite for cross-country running, a high tolerance of mud and more than a passing interest in gadgets, I'm slightly apprehensive about how I'll get on. I'm not as fit as I should be, thanks to a desk job and two exhausting small children. I hope I'll be saved by my secret cartophilia habit (I have been known to download unlabelled maps of the world so I can challenge myself to name each country) and a decent sense of direction. With a bit of luck, this won't end up like The Blair Witch Project and I will make it back to the car before sundown.

Orienteering may sound quite technical, but the basic principle is timing yourself to run courses set through variously challenging terrain. Southdowns Orienteers, a Sussex-based club, race most weekends, and all you need is a pair of supportive running shoes with a good grip and £5.50 to take part. The group does everything else for you – setting a trail through the forest, providing maps and hiring you an electronic dibber for £1, which you use to record your times at electronic control points throughout the course.

We're pretty blessed on this particular Saturday morning in Friston forest, with the misty South Downs as a backdrop and the relief of some warming spring sun. Ali Hooper, an experienced orienteer, takes me through the principles of map reading and route planning, and we follow some short, basic trails using a small compass and colourful maps on A4 paper. Then we measure how many paces we take over 100 metres, and use that to help calculate how far away the next contact point is.

Courses are colour coded for difficulty, from easy white to challenging black. By the time I've done my training, maps for the simpler yellow and orange courses have run out, so I'm left with a 7km green course. I quickly discover it is much, much tougher than my usual cross-country run. Soon I'm wading through mud, scrambling through brambles and thickets and running on spongy leaves. I'm also continually checking my route and direction, and comparing landmarks such as dense undergrowth and mounds against symbols on the map.

Orienteering groups are careful not to over-subscribe events – if you can see the runner ahead of you, it's quite easy to let them do the navigating (or cheat, as some people might call it). I try to ignore Les Hooper, a veteran orienteer of 40 years, but can't help notice that after confidently bounding past him towards control point 6, he has disappeared. Ten minutes later, I'm still inspecting the wrong thicket when I realise Les has long gone. It takes some intense re-examination of my map to work out that I am 30 metres too far east, which perfectly illustrates why map-reading is orienteering's core skill. You don't even have to run; many members, including a 92-year-old in this group, walk routes instead. It can be done at any speed, in any location (there is urban orienteering, ski orienteering, orienteering by horseback – even night orienteering), in any weather and at any age.

What's particularly pleasing is that the whole family could join in too. Les tells me his children and grandchildren are all keen orienteers, and given the perennial concern about screentime, this is a great way of engaging children in a gently competitive and skilled sport that will put a bit of colour on their cheeks. Another seasoned member says he and his wife started orienteering as a last resort – they'd tried many other sports, but he kept winning. They have been orienteering ever since, and she beats him every time.